Separation of 2,7 diisopropylnaphthalene from a feed mixture comprising various diisopropylnaphthalene isomers with a zeolite adsorbent

ABSTRACT

A process for the separation of 2,7 DIPN from a hydrocarbon feed mixture comprising 2,7 DIPN and one or more other DIPN isomers, which process employs an adsorbent, comprising a type Y zeolite, dried to an LOI less than 10% which has been at least partially cation exchanged, at exchangeable sites, with potassium. The 2,7 DIPN thereafter is removed from the adsorbent by contacting it with an aromatic hydrocarbon desorbent material, comprising a xylene, e.g., para-xylene, benzene or toluene at desorbent conditions, and is recovered as an extract product stream. In a preferred embodiment, the process uses a simulated moving-bed countercurrent flow system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 322,329filed Mar. 10, 1989, U.S. Pat. No. 4,929,796.

BACKGROUND OF THE INVENTION

The field of art to which the claimed invention pertains is adsorptiveseparation. More specifically, the invention relates to a process forseparating 2, 7 diisopropylnaphthalene ["DIPN"]from a feed mixturecomprising 2, 7 DIPN and other DIPN isomers, particularly the 2, 6 DIPNisomer, which process employs a particular zeolite adsorbent and aparticular flow scheme to selectively remove and subsequently recoverthe 2, 7 DIPN from the feed mixture.

DESCRIPTION OF THE PRIOR ART

Diisopropylnaphthalene isomers are important chemicals used in theproduction of office copy products such as carbonless paper forms.Heretofore, it has been the practice to produce the various DIPN isomerscatalytically. Of the various isomers, 2, 7 DIPN finds specific utilityin the above application. There is a need, therefore, for isolating aparticular isomer of DIPN rather than utilizing an equilibrium isomericmixture. To my knowledge, there is no present reference in the prior artpertaining to the adsorptive separation of the 2, 7 DIPN from the otherDIPN isomers utilizing therein a zeolitic adsorbent.

It is well known in the separation art that certain crystallinealuminosilicates can be used to separate one hydrocarbon type fromanother hydrocarbon type. The separation of normal paraffins frombranched chain paraffins, for example, can be accomplished by using atype A zeolite which has pore openings from 3 to about 5 Angstroms. Sucha separation process is disclosed in U.S. Pat. No. 2, 985,589 toBroughton et al, and U.S. Pat. No. 3,201,491 to Stine. These adsorbentsallow a separation based on the physical size differences in themolecules by allowing the smaller or normal hydrocarbons to be passedinto the cavities within the zeolitic adsorbent while excluding thelarger or branched chain molecules.

In addition to being used in processes for separating hydrocarbon types,adsorbents comprising type X or Y zeolite have also been employed inprocesses to separate individual hydrocarbon isomers. In the processesdescribed, for example, in U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638to Neuzil, 3,665,046 to deRosset, 3,668,266 to Chen et al, 3,686,343 toBearden Jr. et al, 3,700,744 to Berger et al, 3,894,109 to Rosback and3,997,620 to Neuzil, particular zeolitic adsorbents are used to separatepara-xylene from other xylene isomers. It is also known that theadsorptive capacity of certain zeolites for certain separations isimproved by contacting the zeolite with an aqueous caustic solution. InU.S. Pat. No. 3,374,182 to Young, for example, zeolites so treated aresaid to be effective in separating aromatic hydrocarbons fromnon-aromatic hydrocarbons of the same molecular size. In U.S. Pat. Nos.3,929,669, 3,969,223, and 4,048,111 to Rosback et al, these treatedzeolites are said to be useful for separating olefins from paraffins.

From U.S. Pat. No. 3,888,939 to Rosback, it is also known that reducingthe water content, in terms of loss on ignition (L.O.I.), to less than10% and preferably to from 3-7%, increases the capacity of an adsorbentfor olefins and decreases catalytic activity.

The adsorptive separation of isomers of diisopropylnaphthalene (DIPN)has been disclosed in Japanese Public Disclosure Nos. 243040/88 and243041/88. A two-stage process using two different zeolites is involved,in which 2, 6-DIPN is first coextracted with 2, 7-DIPN and secondly 2,6-DIPN is more strongly adsorbed than 2, 7-DIPN. The adsorbent in eachphase may be Ba-, BaK- or BaPb-Y zeolite and the desorbent is toluene.Japanese Public Disclosures Nos. 243042/88 and 243043/89 disclose thatthe same zeolites just mentioned are used in a two-stage separation ofmethyl isopropylnaphthalene (MIPN) isomers, in which 2, 6-MIPN and 2,7-MIPN are coextracted in the first stage and in which 2, 6-MIPN isrejected in the second stage, i.e., is least strongly adsorbed, while 2,7-MIPN is extracted. The difficulty in predicting adsorptive separationof even similar materials is apparent from these references. Neitherreferences teach applicant's separation wherein 2, 7-DIPN is selectivelyadsorbed on K-Y zeolite.

Japanese Public Disclosure No. 199921/89, dated Aug. 11, 1989, alsodiscloses a two-stage process for separating isomers of DIPN. In thefirst stage, using a first zeolite adsorbent, 1, 7-DIPN and 2, 7-DIPNare extracted. The raffinate from the first stage containing 2, 6-DIPNand other DIPN isomers is fed to a second stage and, using a secondzeolite, 2, 6-DIPN is extracted. The desorbent in both stages isdiethylbenzene or an alkyl benzene containing an n-propyl or isopropylgroup and may also contain one other alkyl group having from 1 to 3carbon atoms.

The present process relates to a process for the separation of 2, 7 DIPNfrom a feed mixture comprising an isomeric mixture of 2, 7 DIPN and oneor more other DIPN isomers. It has been found that adsorbents comprisinga type Y zeolite which has been at least partially exchanged atexchangeable sites with potassium cations exhibit selectivity for 2, 7DIPN and possess other desired characteristics thereby achievingseparation of 2, 7 DIPN by a solid-bed selective adsorption process.

SUMMARY OF THE INVENTION

In brief summary the present invention is, in one embodiment, a processfor separating 2, 7 DIPN from a feed mixture comprising an isomericmixture of 2, 7 DIPN and the other DIPN's, which process comprisescontacting, at adsorption conditions, the feed with an adsorbentcomprising a type Y zeolite, which has been at least partiallyexchanged, at exchangeable sites, with potassium cations, to effect theadsorption of the 2, 7 DIPN and thereafter recovering the 2, 7 DIPN bycontacting the adsorbent with an aromatic hydrocarbon desorbent,selected from the group consisting of toluene, para-xylene, meta-xylene,ortho-xylene or benzene, preferably toluene or para-xylene, atdesorption conditions, thereby desorbing the 2, 7 DIPN to an extractstream.

In another embodiment, the present invention is a process for separating2, 7 DIPN from a feed mixture comprising an isomeric mixture of 2, 7DIPN and the other DIPN's, which process employs an adsorbent comprisinga type Y zeolite, which has been at least partially exchanged, atexchangeable sites, with potassium cations, which process comprises thesteps of: (a) maintaining net fluid flow through a column of theadsorbent in a single direction, which column contains at least threezones having separate operational functions occurring therein and beingserially interconnected with the terminal zones of the column connectedto provide a continuous connection of the zones; (b) maintaining anadsorption zone in the column, the zone defined by the adsorbent locatedbetween a feed input stream at an upstream boundary of the zone and araffinate output stream at a downstream boundary of the zone; (c)maintaining a purification zone immediately upstream from the adsorptionzone, the purification zone defined by the adsorbent located between anextract output stream at an upstream boundary of the purification zoneand the feed input stream at a downstream boundary of the purificationzone; (d) maintaining a desorption zone immediately upstream from thepurification zone, the desorption zone defined by the adsorbent locatedbetween a desorbent input stream at an upstream boundary of the zone andthe extract output stream at a downstream boundary of the zone; (e)passing the feed stream into the adsorption zone at adsorptionconditions to effect the selective adsorption of 2, 7 DIPN by theadsorbent in the adsorption zone and withdrawing a raffinate outputstream from the adsorption zone; (f) passing a desorbent material havinga boiling point different than that of the feed mixture to permitseparation therefrom by distillation into the desorption zone atdesorption conditions to effect the displacement of 2, 7 DIPN from theadsorbent in the desorption zone; (g) withdrawing an extract streamcomprising 2, 7 DIPN and desorbent material from the desorption zone;and (h) periodically advancing through the column of adsorbent in adownstream direction with respect to fluid flow in the adsorption zone,the feed input stream, raffinate output stream, desorbent input stream,and extract output stream to effect the shifting of zones through theadsorbent and the production of extract output and raffinate outputstreams.

Other embodiments of the present invention encompass details about feedmixtures, adsorbents, desorbent materials, flow schemes and operatingconditions all of which are hereinafter disclosed in the followingdiscussion of each of the facets of the present invention.

DESCRIPTION OF THE INVENTION

At the outset the definitions of various terms used throughout thisspecification will be useful in making clear the operation, objects andadvantages of the process.

A "feed mixture" is a mixture containing one or more extract componentsand one or more raffinate components to be fed to an adsorbent of theprocess. The term "feed stream" indicates a stream of feed mixture whichpasses to an adsorbent used in the process.

An "extract component" is a type of compound or a compound, such as anaromatic isomer, that is more selectively adsorbed by the adsorbentwhile a "raffinate component" is a compound or type of compound that isless selectively adsorbed. In this process, 2, 7 DIPN is the extractcomponent and one or more other DIPN isomers is (are) a raffinatecomponent. The term "raffinate stream" or "raffinate output stream"means a stream through which a raffinate component is removed from anadsorbent. The composition of the raffinate stream can vary fromessentially 100% desorbent material (hereinafter defined) to essentially100% raffinate components. The term "extract stream" or "extract outputstream" shall mean a stream through which an extract material which hasbeen desorbed by a desorbent material is removed from the adsorbent. Thecomposition of the extract stream, likewise, can vary from essentially100% desorbent material to essentially 100% extract components. Althoughit is possible by the process of this invention to produce high-purityextract product (hereinafter defined) or a raffinate product(hereinafter defined) at high recoveries, it will be appreciated than anextract component is never completely adsorbed by the adsorbent, nor isa raffinate component completely non-adsorbed by the adsorbent.Therefore, small amounts of a raffinate component can appear in theextract stream, and, likewise, small amounts of an extract component canappear in the raffinate stream. The extract and raffinate streams thenare further distinguished from each other and from the feed mixture bythe ratio of the concentrations of an extract component and a specificcomponent, both appearing in the particular stream. For example, theratio of concentration of the more selectively adsorbed 2, 7 DIPN to theconcentration of less selectively adsorbed other DIPN isomers, will behighest in the extract stream, next highest in the feed mixture, andlowest in the raffinate stream. Likewise, the ratio of the lessselectively adsorbed other DIPN isomers, to the more selectivelyadsorbed 2, 7 DIPN will be highest in the raffinate stream, next highestin the feed mixture, and the lowest in the extract stream. The term"desorbent material" shall mean generally a material capable ofdesorbing an extract component. The term "desorbent stream" or"desorbent input stream" indicates the stream through which desorbentmaterial passes to the adsorbent. When the extract stream and theraffinate stream contain desorbent materials, at least a portion of theextract stream and preferably at least a portion of the raffinate streamfrom the adsorbent will be passed to separation means, typicallyfractionators, where at least a portion of desorbent material will beseparated at separation conditions to produce an extract product and araffinate product. The terms "extract product" and "raffinate product"mean products produced by the process containing, respectively, anextract component and a raffinate component in higher concentrationsthan those found in the respective extract stream and the raffinatestream. The term "selective pore volume" of the adsorbent is defined asthe volume of the adsorbent which selectively adsorbs extract componentsfrom a feed mixture. The term "non-selective void volume" of anadsorbent is the volume of an adsorbent which does not selectivelyretain an extract component from a feed mixture. This volume includesthe cavities of the adsorbent which contain no adsorptive sites and theinterstitial void spaces between adsorbent particles. The selective porevolume and the non-selective void volume are generally expressed involumetric quantities and are of importance in determining the properflow rates of fluid required to be passed into the process for efficientoperations to take place for a given quantity of adsorbent. Whenadsorbent "passes" into an operational zone (which zones are used in apreferred embodiment of this process and are hereinafter defined anddescribed) its non-selective void volume together with its selectivepore volume carries fluid into that zone. The non-selective void volumeis utilized in determining the amount of fluid which should pass intothe same zone in a countercurrent direction to the adsorbent to displacethe fluid present in the non-selective void volume. If the fluid flowrate passing into a zone is smaller than the non-selective void volumerate of adsorbent material passing into that zone, there is a netentrainment of liquid into the zone by the adsorbent. Since this netentrainment is a fluid present in non-selective void volume of theadsorbent, it in most instances comprises less selectively retained feedmixture components. The selective pore volume of an adsorbent can, incertain instances, adsorb portions of raffinate material from the fluidsurrounding the adsorbent since in certain instances there iscompetition between extract material and raffinate material foradsorptive sites within the selective pore volume. If a large quantityof raffinate material with respect to extract material surrounds theadsorbent, raffinate material can be competitive enough to be adsorbedby the adsorbent.

Feed mixtures which can be utilized in the process of this inventionwill comprise isomeric mixtures of 2, 7 DIPN and one or more other DIPNisomers.

The feed mixtures may contain small quantities of straight or branchedchain paraffins, cycloparaffins, or olefinic material It is preferableto have these quantities at a minimum amount in order to preventcontamination of products from this process by materials which are notselectively adsorbed and would be recovered in the raffinate with 2,6-DIPN. Preferably, the above-mentioned contaminants, and especiallythose having boiling points close to 2, 6-DIPN, should be less thanabout 1% of the volume of the feed mixture passed into the process.

To separate 2, 7 DIPN from a feed mixture containing a mixture of 2, 7DIPN and one or more other DIPN isomers, the mixture is contacted withthe particular adsorbent and 2, 7 DIPN is more selectively adsorbed andretained by the adsorbent while the other isomers are relativelyunadsorbed and are removed from the interstitial void spaces between theparticles of adsorbent and the surface of the adsorbent. The adsorbentcontaining the more selectively adsorbed 2, 7 DIPN is referred to as a"rich" adsorbent---rich in the more selectively adsorbed 2, 7 DIPNisomer. The 2, 7 DIPN is then recovered from the rich adsorbent bycontacting the rich adsorbent with a desorbent material.

The desorbent materials which can be used in this process will varydepending on the type of operation employed. The term "desorbentmaterial" as used herein shall mean any fluid substance capable ofremoving a selectively adsorbed feed component from the adsorbent. Inthe swing-bed system in which the selectively adsorbed feed component isremoved from the adsorbent by a purge stream desorbent materialselection is not too critical and desorbent materials comprising gaseoushydrocarbons such as methane, ethane, etc., or other types of gases suchas nitrogen or hydrogen may be used at elevated temperatures or reducedpressures or both to effectively purge the adsorbed feed component fromthe adsorbent. However, in adsorptive separation processes which employzeolitic adsorbents and which are generally operated continuously atsubstantially constant pressures and temperatures so as to maintainliquid phase, the desorbent material relied upon must be judiciouslyselected to satisfy several criteria. First, the desorbent material mustdisplace the extract components from the adsorbent with reasonable massflow rates without itself being so strongly adsorbed as to undulyprevent the extract component from displacing the desorbent material ina following adsorption cycle. Expressed in terms of the selectivity(hereinafter discussed in more detail), it is preferred that theadsorbent be more selective for the extract component with respect to araffinate component than it is for the desorbent material with respectto a raffinate component. Secondly, desorbent materials must becompatible with the particular adsorbent and the particular feedmixture. More specifically, they must not reduce or destroy the criticalselectivity to the adsorbent for the extract components with respect tothe raffinate component. Desorbent materials to be used in the processof this invention should additionally be substances which are easilyseparable from the feed mixture that is passed into the process. Afterdesorbing the extract components of the feed, both desorbent materialand the extract components are typically removed in admixture from theadsorbent. Likewise, one or more raffinate components is typicallywithdrawn from the adsorbent in admixture with desorbent material andwithout a method of separating at least a portion of desorbent material,such as distillation, neither the purity of the extract product nor thepurity of the raffinate product would be very high. It is thereforecontemplated that any desorbent material used in this process will havea substantially different average boiling point than that of the feedmixture to allow separation of desorbent material from feed componentsin the extract and raffinate streams by simple fractionation therebypermitting reuse of desorbent material in the process. The term"substantially different" as used herein shall mean that the differencebetween the average boiling points between the desorbent material andthe feed mixture shall be at least about 5° C. The boiling range of thedesorbent material may be higher or lower than that of the feed mixture.

In the preferred isothermal, isobaric, liquid-phase operation of theprocess of this invention, it has been found that desorbent materialsselected from the group of aromatic hydrocarbons having average boilingpoints substantially different from that of a feed mixture meet thoserequirements and are particularly effective. Aromatic hydrocarbons whichare suitable are toluene, para-xylene, ortho-xylene, meta-xylene andbenzene. Especially preferred for this process is a desorbent materialcomprising substantially pure toluene or para-xylene.

The prior art has recognized that certain characteristics of adsorbentsare highly desirable, if not absolutely necessary, to the successfuloperation of a selective adsorption process. Among such characteristicsare: adsorptive capacity for some volume of an extract component pervolume of adsorbent; the selective adsorption of an extract componentwith respect to a raffinate component and the desorbent material; and,sufficiently fast rates of adsorption and desorption of the extractcomponents to and from the adsorbent.

Capacity of the adsorbent for adsorbing a specific volume of one or moreextract components is, of course, a necessity; without such capacity theadsorbent is useless for adsorptive separation. Furthermore, the higherthe adsorbent's capacity for an extract component, the better is theadsorbent. Increased capacity of a particular adsorbent makes itpossible to reduce the amount of adsorbent needed to separate theextract component contained in a particular charge rate of feed mixture.A reduction in the amount of adsorbent required for a specificadsorptive separation reduces the cost of the separation process. It isimportant that the good initial capacity of the adsorbent be maintainedduring actual use in the separation process over some economicallydesirable life.

The second necessary adsorbent characteristic is the ability of theadsorbent to separate components of the feed; or, in other words, thatthe adsorbent possess adsorptive selectivity, β, for one component ascompared to another component. Relative selectivity can be expressed notonly for one feed component as compared to another but can also beexpressed between any feed mixture component and the desorbent material.The selectivity, B, as used throughout this specification is defined asthe ratio of the two components of the adsorbed phase over the ratio ofthe same two components in the unadsorbed phase at equilibriumconditions.

Relative selectivity is shown as Equation 1 below: ##EQU1## where C andD are two components of the feed represented in volume percent and thesubscripts A and U represent the adsorbed and unadsorbed phasesrespectively. The equilibrium conditions were determined when the feedpassing over a bed of adsorbent did not change composition aftercontacting the bed of adsorbent. In other words, there was no nettransfer of material occurring between the unadsorbed and adsorbedphases.

Where selectivity of two components approaches 1.0 there is nopreferential adsorption of one component by the adsorbent with respectto the other; they are both adsorbed (or non-adsorbed) to about the samedegree with respect to each other. As the β becomes less than or greaterthan 1.0 there is a preferential adsorption by the adsorbent for onecomponent with respect to the other. When comparing the selectivity bythe adsorbent of one component C over component D, a β larger than 1.0indicates preferential adsorption of component C within the adsorbent. Aβ less than 1.0 would indicate that component D is preferentiallyadsorbed leaving an unadsorbed phase richer in component C and anadsorbed phase richer in component D. While separation of an extractcomponent from a raffinate component is theoretically possible when theselectivity of the adsorbent for the extract component with respect tothe raffinate component just exceeds a value of 1.0, it is preferredthat such selectivity have a value approaching or exceeding 2. Likerelative volatility, the higher the selectivity the easier theseparation is to perform. Higher selectivities permit a smaller amountof adsorbent to be used in the process. Ideally desorbent materialsshould have a selectivity equal to about 1 or less than 1 with respectto all extract components so that all of the extract components can beextracted as a class and all raffinate components clearly rejected intothe raffinate stream.

The third important characteristic is the rate of exchange of theextract component of the feed mixture material or, in other words, therelative rate of desorption of the extract component. Thischaracteristic relates directly to the amount of desorbent material thatmust be employed in the process to recover the extract component fromthe adsorbent; faster rates of exchange reduce the amount of desorbentmaterial needed to remove the extract component and therefore permit areduction in the operating cost of the process. With faster rates ofexchange, less desorbent material has to be pumped through the processand separated from the extract stream for reuse in the process.

In order to test various adsorbents and desorbent material with aparticular feed mixture to measure the adsorbent characteristics ofadsorptive capacity and selectivity and exchange rate a dynamic testingapparatus is employed. The apparatus consists of an adsorbent chamber ofapproximately 70 cc volume having inlet and outlet portions at oppositeends of the chamber. The chamber is contained within a temperaturecontrol means and, in addition, pressure control equipment is used tooperate the chamber at a constant predetermined pressure.Chromatographic analysis equipment can be attached to the outlet line ofthe chamber and used to analyze "on-stream" the effluent stream leavingthe adsorbent chamber.

A pulse test, performed using this apparatus and the following generalprocedure, is used to determine selectivities and other data for variousadsorbent systems. The adsorbent is filled to equilibrium with aparticular desorbent by passing the desorbent material through theadsorbent chamber. At a convenient time, a pulse of feed containingknown concentrations of a non-adsorbed linear paraffinic tracer (n-C₁₄,for instance) and of the particular feed isomers all diluted indesorbent is injected for a duration of several minutes. Desorbent flowis resumed, and the tracer and the isomers are eluted as in aliquid-solid chromatographic operation. The effluent can be analyzed byon-stream chromatographic equipment and traces of the envelopes ofcorresponding component peaks developed. Alternately, effluent samplescan be collected periodically and later analyzed separately be gaschromatography.

From information derived from the chromatographic traces, adsorbentperformance can be rated in terms of capacity index for an extractcomponent, selectivity for one isomer with respect to the other, and therate of desorption of an extract component by the desorbent. Thecapacity index may be characterized by the distance between the centerof the peak envelope of the selectively adsorbed isomer and the peakenvelope of the tracer component of some other known reference point. Itis expressed in terms of the volume in cubic centimeters of desorbentpumped during this time interval. Selectivity, β, for an extractcomponent with respect to a raffinate component may be characterized bythe ratio of the distance between the center of an extract componentpeak envelope and the tracer peak envelope (or other reference point) tothe corresponding distance between the center of a raffinate componentpeak envelope and the tracer peak envelope. The rate of exchange of anextract component with the desorbent can generally be characterized bythe width of the peak envelopes at half intensity. The narrower the peakwidth the faster the desorption rate. The desorption rate can also becharacterized by the distance between the center of the tracer peakenvelope and the disappearance of an extract component which has justbeen desorbed. This distance is again the volume of desorbent pumpedduring this time interval.

Adsorbents to be used in the process of this invention will comprisespecific crystalline aluminosilicates or molecular sieves. Particularcrystalline aluminosilicates encompassed by the present inventioninclude crystalline aluminosilicate cage structures in which the aluminaand silica tetrahedra are intimately connected in an open threedimensional network. The tetrahedra are cross-linked by the sharing ofoxygen atoms with spaces between the tetrahedra occupied by watermolecules prior to partial or total dehydration of this zeolite. Thedehydration of the zeolite results in crystals interlaced with cellshaving molecular dimensions. Thus, the crystalline aluminosilicates areoften referred to as "molecular sieves" when the separation which theyeffect is dependent essentially upon differences between the sizes ofthe feed molecules as, for instance, when smaller normal paraffinmolecules are separated from larger isoparaffin molecules by using aparticular molecular sieve. In the process of this invention, however,the term "molecular sieves," although widely used, is not strictlysuitable since the separation of the isomers of DIPN is apparentlydependent on differences in electrochemical attraction of the differentDIPN isomers on the adsorbent as well as on the size differences of theDIPN isomer molecules.

In hydrated form, the crystalline aluminosilicates generally encompassthose zeolites represented by the Formula 1a below:

    M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O       Formula 1a

where "m" is a cation which balances the electrovalence of thetetrahedra and is generally referred to as an exchangeable cationicsite, "n" represents the valence of the cation, "w" represents the molesof SiO₂, and "y" represents the moles of water. The generalized cation"M" may be monovalent, divalent or trivalent cations or mixturesthereof.

The prior art has generally recognized that adsorbents comprising thetype X and the type Y zeolites can be used in certain adsorptiveseparation processes. These zeolites are well known to the art.

The type X structured zeolite in the hydrated or partially hydrated formcan be represented in terms of mole oxides as shown in Formula 2 below:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5±0.5)SiO.sub.2 :yH.sub.2 OFormula 2

where "M" represents at least one cation having a valence of not morethan 3, "n" represents the valence of "M" and "y" is a value up to about9 depending upon the identify of "M" and the degree of hydration of thecrystal. As noted from Formula 2 the SiO₂ /Al₂ O₃ mole ratio is 2.5±0.5.The cation "M" may be one or more of a number of cations such as thehydrogen cation, the alkali metal cation, or the alkaline earth cations,or other selected cations, and is generally referred to as anexchangeable cationic site. As the type X zeolite is initially prepared,the cation "M" is usually predominantly sodium and the zeolite istherefore referred to as a sodium type X zeolite. Depending upon thepurity of the reactants used to make the zeolite, other cationsmentioned above may be present, however, as impurities.

The type Y structured zeolite in the hydrated or partially hydrated formcan be similarly represented in terms of mole oxides as in Formula 3below:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :sWiO.sub.2 :H.sub.2 OFormula 3

where "M" is at least one cation having a valence not more than 3, "n"represents the valence of "M", "w" is a value greater than about 3 up to6, and "y" is a value up to about 9 depending upon the identity of "M"and the degree of hydration of the crystal. The SiO₂ /Al₂ O₃ mole ratiofor type Y structured zeolites can thus be from about 3 to about 6. Likethe type X structured zeolite, the cation "M" may be one or more of avariety of cations but, as the type Y zeolite is initially prepared, thecation "M" is also usually predominantly sodium. The type Y zeolitecontaining predominantly sodium cations at the exchangeable cationicsites is therefore referred to as a sodium type Y zeolite.

Cations occupying exchangeable cationic sites in the zeolite may bereplaced with other cations by ion exchange methods generally known tothose having ordinary skill in the field of crystallinealuminosilicates. Such methods are generally performed by contacting thezeolite or a base material containing the zeolite with an aqueoussolution of the soluble salt of the cation or cations desired to beplaced upon the zeolite. After the desired degree of exchange takesplace the sieves are removed from the aqueous solution, washed and driedto a desired water content. By such methods the sodium cations and anynon-sodium cations which might be occupying exchangeable sites asimpurities in a sodium type X or sodium type Y zeolite can be partiallyor essentially completely replaced with other cations.

The term "base material" as used herein shall refer to a materialcontaining an X or a Y zeolite and amorphous material which can be usedto make the adsorbents used in my process. The zeolite will typically bepresent in the base material in amounts ranging from about 75 wt.% toabout 98 wt.% of the base material based on volatile free composition.Volatile free compositions are generally determined after the basematerial has been calcined at 900° C. in order to drive off all volatilematter. The remainder of the base material will generally be amorphousmaterial such as silica, alumina or silica alumina mixtures orcompounds, such as clays, which material is present in intimate mixturewith the small particles of the zeolite material. This amorphousmaterial may be an adjunct of the manufacturing process for X or Yzeolite (for example, intentionally incomplete purification of eitherzeolite during its manufacture) or it may be added to relatively pure Xor Y zeolite but in either case its usual purpose is as a binder to aidin forming or agglomerating the hard crystalline particles of thezeolite. Normally the base material will be in the form of particlessuch as extrudates, aggregates, tablets, macrospheres or granules havinga desired particle size range. The adsorbent used in this process willpreferably have a particle size range of about 16-60 mesh (Standard U.S.Mesh). Examples of suitable base materials which can be used to make theadsorbents employed in my process can be obtained from commercialsources such as the Linde Division of Union Carbide Corporation.

It has been found that an adsorbent comprising a type Y zeolitecontaining at exchangeable cationic sites potassium cations satisfiesthe selectivity requirements and other adsorbent requirements previouslydiscussed and are therefore preferred for use in this process.Adsorbents for this process may be prepared by at least partially ionexchanging sodium type Y base material, in a particle size range of fromabout 20 to about 40 U.S. mesh, with the above-mentioned selectedcation. Within the scope of the present invention, it should berecognized however that ion exchange may proceed to essentialcompletion. Typically the ion exchanges will be done with aqueoussolutions of the soluble salts, such as the chlorides, of the respectivemetal. The term "essential completion" shall mean that the residualsodium content of the adsorbent after the ion exchange of the basematerial shall be less than about 2 wt. % Na₂ O. After ion-exchange andwater wash to remove excess ion exchange solution, the adsorbent willtypically be dried to reduce the water content as measured by loss onignition (LOI) at 900° C. to less than about 10 wt. % and morepreferably within a range of from about 1 to about 5 wt. %. For adiscussion of the effect of water content, reference is made to RosbackU.S. Pat. No. 3,888,939.

The adsorbent may be employed in the form a dense fixed bed which isalternately contacted with a feed mixture and a desorbent material inwhich case the process will be only semicontinuous. In anotherembodiment a set of two or more static beds of adsorbent may be employedwith appropriate valving so that a feed mixture can be passed throughone or more adsorbent beds of a set while a desorbent material can bepassed through one or more of the other beds in a set. The flow of afeed mixture and a desorbent material may be either up or down throughan adsorbent in such beds. Any of the conventional apparatus employed instatic bed fluid-solid contacting may be used.

Separation processes employing countercurrent moving-bed or simulatedmoving-bed countercurrent flow systems, however, have much greaterseparation efficiencies than do separation processes employing fixedadsorbent bed systems. With the moving-bed or simulated moving-bed flowsystems a feed mixture and a desorbent material are continuously fed tothe process and adsorption and desorption are continuously taking placewhich allows continuous production of an extract output stream and araffinate output stream. In a preferred embodiment therefore the processwill use such flow systems. In a more preferred embodiment the processwill employ a simulated moving-bed countercurrent flow system. Theoperating principles and sequence of operation of one such simulatedmoving-bed countercurrent flow system are described in U.S. Pat. No.2,985,589 incorporated herein by reference. In such a system it is theprogressive movement of multiple liquid access points down an adsorbentchamber that simulates the upward movement of an adsorbent contained inthe chamber. Only four of the access lines are active at any one time;the feed input stream, desorbent inlet stream, raffinate outlet stream,and extract outlet stream access lines. Coincident with this simulatedupward movement of a solid adsorbent is the movement of a liquidoccupying the void volume of the packed bed of adsorbent. So thatcountercurrent contact is maintained, a liquid flow down the adsorbentchamber may be provided by a pump. As an active liquid access pointmoves through a cycle, that is, from the top of the chamber to thebottom, the chamber circulation pump moves through different zones whichrequired different flow rates. A programmed flow controller may beprovided to set and regulate these flow rates.

The active liquid access points effectively divided the adsorbentchamber into separate zones, each of which has a different function. Inthis embodiment of my process it is generally necessary that threeseparate operational zones be present in order for the desiredoperations to take place although in some instances an optional fourthzone may be used.

The adsorption zone, zone 1, is defined as the adsorbent located betweena feed inlet stream and raffinate outlet stream. In this zone, a feedmixture contacts an adsorbent, an extract component is adsorbed, and araffinate stream is withdrawn. Since the general flow through zone 1 isfrom the feed stream which passes into the zone to the raffinate streamwhich passes out of the zone, the flow in this zone is considered to bea downstream direction when proceeding from the feed inlet to theraffinate outlet streams.

Immediately upstream with respect to fluid flow in zone 1 is thepurification zone, zone 2. The purification zone is defined as theadsorbent between an extract outlet stream and the feed inlet stream.The basic operations taking place in zone 2 are the displacement fromthe non-selective void volume of the adsorbent of any raffinate materialcarried into zone 2 by the shifting of adsorbent into this zone and thedesorption of any raffinate material adsorbed within the selective porevolume of the adsorbent or adsorbed on the surfaces of the adsorbentparticles. Purification is achieved by passing a portion of extractstream material leaving zone 3 (hereinafter described) into zone 2 atzone 2,'s upstream boundary, the extract outlet stream, to effect thedisplacement of raffinate material. The flow of material in zone 2 is ina downstream direction from the extract outlet stream to the feed inletstream.

Immediately upstream of zone 2 with respect to the fluid flowing in zone2 is the desorption zone or zone 3. The desorption zone is defined asthe adsorbent between a desorbent inlet stream and the extract outletstream. The function of the desorption zone is to allow a desorbentmaterial which passes into this zone to displace the extract componentwhich was adsorbed upon the adsorbent during a previous contact withfeed in zone 1 in a prior cycle of operation. The flow of fluid in zone3 is essentially in the same direction as that of zones 1 and 2.

In some instances an optional buffer zone, zone 4, may be utilized. Thiszone, defined as the adsorbent between the raffinate outlet stream andthe desorbent inlet stream, if used, is located immediately upstreamwith respect to the fluid flow to zone 3. Zone 4 would be utilized toconserve the amount of desorbent utilized in the desorption step since aportion of the raffinate stream which is removed from zone 1 can bepassed into zone 4 to displace desorbent material present in that zoneout of that zone into the desorption zone. Zone 4 will contain enoughadsorbent so that raffinate material present in the raffinate streampassing out of zone 1 and into zone 4 can be prevented from passing intozone 3 thereby contaminating the extract stream removed from zone 3. Inthe instances in which the fourth operational zone is not utilized theraffinate stream passed from zone 1 to zone 4 must be carefullymonitored in order that the flow directly from zone 1 to zone 3 can bestopped when there is an appreciable quantity of raffinate materialpresent in the raffinate stream passing from zone 1 into zone 3 so thatthe extract outlet stream is not contaminated.

A cyclic advancement of the input and output streams through the fixedbed of an adsorbent can be accomplished by utilizing a manifold systemin which the valves in the manifold are operated in a sequential mannerto effect the shifting of the input and output streams thereby allowinga flow of fluid with respect to solid adsorbent in a countercurrentmanner. Another mode of operation which can effect the countercurrentflow of solid adsorbent with respect to fluid involves the use of arotating disc valve in which the input and output streams are connectedto the valve and the lines through which feed input, extract output,desorbent input and raffinate output streams pass are advanced in thesame direction through the adsorbent bed. Both the manifold arrangementand disc valve are known in the art.

Specifically rotary disc valves which can be utilized in this operationcan be found in U.S. Pat. Nos. 3,040,777 and 3,422,848, incorporatedherein by reference. Both of the aforementioned patents disclose arotary type connection valve in which the suitable advancement of thevarious input and output streams from fixed sources can be achievedwithout difficulty.

In many instances, one operational zone will contain a much largerquantity of an adsorbent than some other operational zone. For instance,in some operations the buffer zone can contain a minor amount of anadsorbent as compared to the adsorbent required for the adsorption andpurification zones. It can also be seen that when a very efficientdesorbent material is used which can easily desorb an extract componentfrom an adsorbent, it is possible that a relatively small amount ofadsorbent will be needed in a desorption zone as compared to theadsorbent needed in the buffer zone or adsorption zone or purificationzone. It is not required that an adsorbent be located in a single columnwhich is divided into zones, and the use of multiple chambers or aseries of columns is also within the scope of this embodiment.

It is not necessary that all of the input or output streams besimultaneously used, and, in fact, in many instances, some of thestreams can be shut off while others effect an input or output ofmaterial. One apparatus which can be utilized to effect the process ofthis invention in a preferred embodiment will contain a series ofindividual beds connected by connecting conduits upon which are placedinput or output taps to which the various input or output streams can beattached and alternately and periodically shifted to effect continuousoperation. In some instances, the connecting conduits can be connectedto transfer taps which during the normal operations functionintermittently as a conduit through which material passes into or out ofthe process.

It is contemplated that at least a portion of the extract output streamwill pass into a separation means wherein at least a portion of thedesorbent material can be separated at separating conditions to producean extract product containing a reduced concentration of desorbentmaterial. Preferably, but not necessary to the operation of the process,at least a portion of the raffinate output stream will also be passed toa separation means wherein at least a portion of the desorbent materialcan be separated at separating conditions to produce a desorbent streamwhich can be reused in the process and a raffinate product containing areduced concentration of desorbent material. Typically the concentrationof desorbent material in the extract product and the raffinate productwill be less than about 5 vol. % and more preferably less than about 1vol. %. The separation means will typically be a fractionation column,the design and operation of which is well known to the separation art.

Reference can be made to D. B. Broughton U.S. Pat. No. 2,985,589 and toa paper entitled "Continuous Adsorptive Processing--A New SeparationTechnique" by D. B. Broughton presented at the 34th Annual Meeting ofthe Society of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969, forfurther explanation of the simulated moving bed countercurrent processflow scheme.

Although both liquid and vapor phase operations can be used in manyadsorptive separation processes, liquid-phase operation is preferred forthis process because of the lower temperature requirements and becauseof the higher yields of an extract product that can be obtained withliquid-phase operation over those obtained with vapor-phase operation.Adsorption conditions will include a temperature range of from about 20°C. to about 250° C. with about 150° C. to about 180° C. being morepreferred and a pressure range of from about atmospheric to about 500psig with from about atmospheric to about 250 psig being more preferredto insure liquid phase. Desorption conditions will include the samerange of temperatures and pressure as used for adsorption conditions.

The size of the units which can utilize the process of this inventioncan vary anywhere from those of pilot-plant scale (see for example U.S.Pat. No. 3,706,812) to those of commercial scale and can range in flowrates from as little as a few cc an hour up to many thousands of gallonsper hour.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a plot of the data obtained during the conduct of the example,described hereinbelow.

The following example is presented for illustration purposes and morespecifically to illustrate the selectivity relationships that make theprocess of the invention possible. Reference to specific cations,desorbent materials, feed mixtures and operating conditions is notintended to unduly restrict the scope and spirit of the claims attachedhereto.

EXAMPLE I

In this experiment a pulse test was performed to evaluate the ability ofan adsorbent to separate a feed mixture containing DIPN isomers. Thebase material contained a sodium form Linde type Y structured zeoliteand a small portion of sodium form amorphous binder material. It wasion-exchanged with potassium cations, to give an adsorbent whichcontained less than about 1 wt. % residual sodium as Na₂ O. Theadsorbent was dried to approximately 1 wt % LOI at 500° C. before it wasutilized in the process.

The testing apparatus was an adsorbent chamber containing approximately70 cc of the adsorbent and contained with a temperature-controlled meansin order to maintain essentially isothermal operations through thecolumn. For the pulse test the column was maintained at a temperature of170° C. and a pressure of 100 psig to maintain liquid-phase operations.Gas chromatographic analysis equipment was attached to the columneffluent stream in order to determine the composition of the effluentmaterial at given time intervals. DIPN isomers, other than 2, 6- and 2,7-DIPN, were not specifically identified. The feed mixture employed forthe test contained about 19 vol. % of a commercial DIPN isomer blendalong with 3 vol. % of n-C₁₄, which was used as a tracer, less than 1vol. % light materials and triisopropylnaphthalenes (TIPN's) and 77 vol.% desorbent material. The desorbent material was run continuously at anominal liquid hourly space velocity (LHSV) of 1.0. At some convenienttime interval the desorbent was stopped and the feed mixture wasinjected at an LHSV of 1.0. The desorbent stream was then resumed at 1LHSV and continued to pass into the adsorbent column until all of thefeed isomers had been eluted from the column as determined by monitoringthe chromatograph generated by the effluent material leaving theadsorption column. The sequence of operations usually takes about anhour. The pulses of feed and subsequent desorption may be repeated insequence as often as is desired.

The attached FIG. 1 comprises the chromatographic traces of thepulse-tested adsorbent, showing the relative selectivities for 2, 7DIPN, for the other DIPN isomers, the light material and tracer elutedfrom the column. Further, selected information derived from thesetraces, such as selectivity, (β) and net retention value (NRV) isindicated in the following table.

                  TABLE 1                                                         ______________________________________                                        Component         NRV     β                                              ______________________________________                                        n-C14             0.0     tracer                                              Lights + TIPN's   4.4     2.84                                                DIPN ISOMER A     3.4     3.68                                                DIPN ISOMER B     0.8     15.6                                                DIPN ISOMER C     11.7    1.07                                                2,6 DIPN          1.8     6.94                                                2.7 DIPN          12.5    Reference                                           ______________________________________                                    

EXAMPLE II

Another pulse test was performed to evaluate the separation of ExampleI, under the same conditions, using a different desorbent, p-xylene. Thefeed pulse was 5 ml. of a mixture of 1.5 ml. of the same commercial DIPNisomer blend used in Example I containing lights and TIPN's, 0.5 ml.n-C₁₄ and 5 m. of the desorbent, p-xylene. The results of the separationare shown in the following Table 2, whereby 2, 6- DIPN can be recoveredin the raffinate and 2, 7-DIPN can be recovered in the extract stream.Lights and TIPN's in the effluent stream were not analyzed in the pulsetest and all other DIPN isomers were separated together.

                  TABLE 2                                                         ______________________________________                                        Component       GRV     NRV       β                                      ______________________________________                                        n-C.sub.14      45.6     0.0      (tracer)                                    2,6-DIPN        48.7     3.1      6.13                                        2,7-DIPN        64.6    19.0      1.00 (ref.)                                 Other DIPN Isomers                                                                            59.1    13.5      1.41                                        ______________________________________                                    

What is claimed:
 1. A process for separating 2, 7 diisopropylnaphthalene(DIPN) from a feed mixture comprising, 2, 7 DIPN and at least one otherDIPN isomer, which process comprises contacting, at adsorptionconditions, said feed with an adsorbent comprising a Y zeolite which hasbeen at least partially exchanged at exchangeable sites, with potassiumcations, to effect the adsorption of 2, 7 DIPN and thereafter recoveringthe 2, 7 DIPN by desorption, under desorption conditions, with adesorbent, comprising an aromatic hydrocarbon.
 2. The process of claim 1wherein said desorbent is selected from the group consisting of toluene,para-xylene, ortho-xylene, meta-xylene and benzene.
 3. The process ofclaim 1 wherein said desorbent is para-xylene.
 4. The process of claim 1wherein said desorption conditions include a temperature within therange of from about 20° C. to about 250° C. and a pressure within therange of from about atmospheric to about 500 psig so as to maintainliquid phase.